Many diseases result from damage, malfunction, or loss of a single organ or tissue type (1). While certain strategies such as organ transplants can be effective, the demand for replacement organs far exceeds availability, resulting in an average of 18 deaths per day in the US alone (3). The development of engineered tissues count among the most promising multidisciplinary approaches to fulfill this demand (1,4). However, formation of large tissue constructs with sufficient cell mass to replace critical organ functions can be hampered by the diffusion limit of oxygen and nutrients to cells within the construct. Based on the rates of oxygen exchange, transport of nutrients and secreted factors, and waste removal, it is estimated that cells must be located within 150-200 μm of the nearest capillary blood vessel to survive and function optimally (5, 6). As such, certain efforts to implant large engineered tissue structures are hindered by significant cell death in areas that exceed this diffusion limit within hours to days of implantation.
Although some tissues can function with lower capillary densities, adequate perfusion of metabolically active tissue requires intimate localization of parenchymal cells to a dense vasculature in a highly organized manner (21, 32, 19). For example, the liver has a precisely defined organization in which hepatocytes and microvessels are interdigitated in a highly aligned microarchitecture (24, 25). In addition, the architecture of the vasculature itself, e.g., the branching frequency and angles, alignment of vessels, and tortuosity, constrains gradients of metabolite exchange and the overall flow fields through the tissue. Therefore, the engineering of such tissues can require approaches to define the geometric architecture of vascular networks for tissue-specific applications.
Certain cell-based pre-vascularization strategies of engineered tissues have utilized randomly seeded cells embedded within a three dimensional matrix. For example, investigators discovered that the speed of vascularization can be increased by allowing endothelial cells to form rudimentary networks in vitro prior to implantation (9). It has been demonstrated that implantation of scaffolds pre-seeded with endothelial cells (ECs) facilitates tubulogenesis (the formation of interconnected web-like networks of interconnected endothelial cells) within the scaffold and eventual anastomosis (connection) of the newly formed tubules to host vessels within days to weeks (6-9). Unfortunately, such networks are randomly distributed, and it is difficult to control the formation and structure of vessels in a fixed and reproducible manner. For example, the random organization of endothelial networks provides no directional guidance to incoming host vessels, often resulting in only an outer shell of the implant becomes perfused, leaving the interior core under-perfused. Furthermore, the strict spatial organization of cells, the surrounding extracellular matrix (ECM), and vasculature can impact paracrine signaling gradients that define cellular phenotypes and tissue function [10]. Therefore, the ability to precisely control the 3D geometry of vascular networks is important for the development of therapeutic tissue engineering constructs.
Although the field of tissue engineering has made progress since its inception, a question remains regarding how to rapidly and adequately vascularize and integrate an engineered tissue with host tissue. Although certain techniques have emerged as potential solutions (11, 12), and approaches that include cells have shown some promise in enhancing tissue integration, they have focused on injecting cells directly into a site or a randomly organized suspension of cells inside a biomaterial that can form small, randomly dispersed networks of cells. In addition, certain studies have not demonstrated the ability to rapidly create long-lasting vessels with the capability of long-term support of parenchymal cells (6, 13).